The present invention relates to a formable multilayer polymeric body which exhibits image sharpness and a high degree of reflectance over substantially the entire visible region of the spectrum (i.e., substantially colorless), and to articles produced therefrom which may find use as mirrors, reflectors and other reflective parts and trim, lenses, polarizers, and the like.
Conventional methods for fabricating reflective surfaces include forming such surfaces of highly polished metals. Because of the high costs and fabricating problems involved in using metals, more recently fabricators have used plastic surfaces which contain thin coatings of metal thereon. Thus, metal coated plastic articles are now commonly found as both decorative and functional items in a number of industries including commercial and industrial lighting applications. Such articles are used as bright work for consumer appliances such as refrigerators, dishwashers, washers, dryers, radios, and the like. These types of articles are also used by the automotive industry as head lamp reflectors, bezels, radio knobs, automotive trim, and the like.
Typically, such metal coated plastic articles are formed by electroplating or by the vacuum, vapor, or chemical deposition of a thin metal layer on the surface of the article. It has been observed that such coatings are subject to chipping and flaking of the metal as well as corrosion of the metal over time. If additional protective layers must be applied over the metal coating to protect it, additional labor and materials costs are involved. Further, there may be environmental disposal problems with some metal deposition processes.
Thermoforming equipment is widely available and capable of being readily adapted to form a variety of shapes and finished parts from sheets of thermoplastic material. One major drawback in the use of metal coated plastic articles is that the metal must be deposited on a plastic surface which has been previously shaped into a finished article. This limits the users of such processes to those who have the capability of vacuum or other metallization techniques. Thus, sheets of metallized thermoplastic cannot generally be thermoformed into articles requiring significant draw.
Multilayer articles of polymers are known, as are methods and apparatuses for making such articles. For example, such multilayered articles may be prepared utilizing multilayer coextrusion devices as described in commonly-assigned U.S. Pat. Nos. 3,773,882 and 3,884,606 to Schrenk. Such devices are capable of simultaneously extruding diverse thermoplastic polymeric materials in substantially uniform layer thicknesses. The number of layers may be multiplied by the use of a device as described in commonly-assigned U.S. Pat. No. 3,759,647 to Schrenk et al.
Alfrey, Jr. et al, U.S. Pat. No. 3,711,176, teaches a multilayered highly reflective thermoplastic body fabricated using optically thin film techniques. That is, the reflective thin film layers of Alfrey, Jr. et al rely on the constructive interference of light to produce reflected visible, ultraviolet, or infrared portions of the electromagnetic spectrum. Such reflective thin films have found use in decorative items because of the iridescent reflective qualities of the film. See also, Cooper, U.S. Pat. No. Re. 31,780.
However, the films of Alfrey, Jr. are extremely sensitive to thickness changes, and it is characteristic of such films to exhibit streaks and spots of nonuniform color. Further, color reflected by such films is dependent on the angle of incidence of light impinging on the film. Thus, such films are not practical for uses which require uniformity of reflectivity (i.e., colorless and substantially noniridescent). Moreover, such films are not practical to thermoform into articles as localized thinning of the layers during thermoforming causes alterations in the iridescent coloring and reflective characteristics of the films.
Radford et al, "Reflectivity of Iridescent Coextruded Multilayered Plastic Films", Polymer Engineering and Science, vol. 13, No. 3 (May 1973), proposed introducing a linear layer thickness gradient into a multilayer optically thin film to produce a film which would reflect the majority of the visible spectrum and exhibit a metallic appearance. Again, however, the films proposed by Radford et al would not be practical to thermoform into articles as localized thinning of the layers during thermoforming would cause alterations in the reflective characteristics of the films and induce iridescent colors.
More recently, multilayer coextrusion technology has been used to fabricate reflective multilayer films and sheets made up of optically thick layers (optical thickness of greater than about 0.45 Nm) or a combination of optically thick and optically very thin layers (optical thickness of less than about 0.09 Nm). See, Wheatley et al, U.S. Pat. No. 5,122,905, Wheatley, U.S. Pat. No. 5,122,906, and Wheatley et al, U.S. Pat. No. 5,126,880. The latter describes a uniformly reflective all polymer body in which the iridescent effects from optically thin layers are masked.
While such multilayer bodies reflect substantially uniformly over the entire visible spectrum and are essentially colorless and noniridescent, there are drawbacks to their use in the thermoforming of articles. To maintain high reflectivity, relatively thick sheets (e.g., 2.5 mm or thicker) of a thousand or more layers must be used. Drawing of such sheets during thermoforming, even for relatively low draw ratios of 2:1 or less, also may bring on the onset of undesirable iridescence as optically thick layers in the sheet are thinned into the optically thin range. Even when drawn, the sheet thickness may still be greater than that desired for a particular application.
Accordingly, the need still exists in this art for a formable multilayer polymeric body which exhibits image sharpness and has a high substantially uniform reflectivity in the visible region of the spectrum and which can maintain such uniform reflectivity during drawing conditions typically encountered in thermoforming operations.